As is well known to those skilled in the art, various kinds of paper are used nowadays for photocopiers. These papers or more broadly sheets are, however, classified mainly into two categories. One is the common class paper and the other is the special class paper. The common class papers herein referred to as such, are most commonly used in citizens' daily life. This kind of paper has a relatively rough surface on which, generally, additives thereto remain in powder-like state and as deposits thereon. During passage between and through a pair of feed rollers, it is believed that these additives will act as rolling medium to a certain degree, thus providing a lesser friction coefficient similar to rolling one, relative to the rotating feed rollers. The special class papers include overhead projector paper, briefly referred to herein as OHP-sheet; secondary original drawing sheet; offset printing master sheet and the like, having a relatively smooth surface and remaining on the surface substantially no powder-like additives. During use, this kind of paper is believed to provide a sliding friction coefficient through and between a pair of feed rollers. Therefore, the coefficient amounts to a relatively high value.
It is demanded for the paper feed mechanism used in the photoelectric copier machine to use not only the comnon class papers, but also special class papers as specifically referred to above, and indeed, to provide a highly stabilized feedability.
In the case of such a paper feed mechanism arranged at a downstream position when seeing in the paper feed direction and comprising a regularly revolving feed roller and a normally reversedly rotating paging roller as a mating member cooperating therewith, when the latter roller is driven in the reversed sense which means such rotational direction as returning the sheet(s) in the reversing direction, while the feed roller is kept always in regular rotation in paper feeding direction, for feeding a sheet of the special class paper, having thus a considerably high friction coefficient, failure in paper feed has occurred rather frequently, thereby constituting a grave drawback in the art.
For dissolving such a conventional drawback in conventional art, a torque limiter was fitted to the paging roller, and indeed, for such purpose as to provide a stabilized paper feed performance, even in use of special class paper sheets having a high friction coefficient as was referred to hereinabove, while, on the other hand, always assuring the reliability in the paging roller performance.
As an example thereof, Japanese Open Utility Model Publication No. (unexamined) Sho-60-47741 may be raised. In this specific prior art, a torque limiter is fitted to the paging roller and as the material for the both mutually mating rollers, polynorbonene rubber representing a superior antifrictional performance is utilized for molding purpose for these rollers.
With provision of such torque limiter attached to in the aforementioned way, and if a paper sheet is fed at the nip of these mating rollers, the paging roller will rotate in the paper feed executing direction by virtue of the follower like motion of the paging roller under the influence of the sheet-carrying force provided by the regularly revolving feed roller. If, however, two or more paper sheets are introduced between these rollers, the paging roller will be caused to reverse its rotational direction for urging the second and further sheets, if any, to make a return movement towards the paper feed cassette. Therefore, it will be seen that even if a sheet of special class paper, having a high friction coefficient should be introduced into the nip portion between the both rollers, troubles in paper feed operation could not be encountered.
On the other hand, we have experienced, however, such that the paging mechanism, even if fitted with torque-limiting means represents several drawbacks, as will be later more fully described with reference to the drawings.